1. Field of the Invention
The invention relates to a sealing arrangement, particularly including a seal with automatic clearance adjustment.
2. Description of the Background
This seal forms part of the family of segmented seals composed of elements called segments that are butt-jointed and each extend over a portion of the circumference; these segments are connected together by a surrounding spring that tends to force them into contact with a part such as a shaft with which the seal is to be formed.
The problem that arises in this case is wear of the seal caused by friction due to the shaft rotating on the seal; since this wear is proportional to the shaft rotation speed and the pressure of the segments, it is desirable to minimize this pressure. One method would be to suitably choose the spring or to adjust its characteristics, particularly making sure that it is not too tight, but in reality this would not be very easy or even very useful since this force is very small compared with the radial pressure force exerted on the segments. Another solution is to use the rotation speed of the rotor to create a hydro-dynamic air pressure at the contact between segments and the rotor, subject to some arrangements on segments (Rayleigh pads). The major disadvantage of this technology is that it is inefficient at low and medium rotor speeds. Therefore, up to now there has been no efficient way of limiting wear of segmented seals, which explains why they only last for a few thousand hours which is quite inadequate for most applications.
The arrangement proposed in this description is characterized by an automatic reduction in the pressure at which seal segments are forced into contact with the friction part, and possibly elimination of this pressure. In its most general form, the sealing arrangement described in this description comprises a seal composed of butt jointed segments in the shape of an arc of a circle and a surrounding spring joining the segments together, the segments having a sealing face on the side opposite the spring and being laid out in contact with a part passing through an opening in a wall separating a high pressure containment from a low pressure containment; this arrangement is characterized in that the seal is put into position in front of the opening with high pressure applied to a first portion of one face opposite the sealing face and low pressure applied to a second portion of the face opposite the sealing face, and in that the sealing faces are shaped to form chambers surrounded by a thrust lip acting on the part, orifices passing through the segments from the chambers as far as the high pressure containment. As will be seen, the orifices (which may be calibrated to a given diameter, for example 1 millimeter) enable a pressure to be set up in the chambers to reduce the tightness of the segments. Preferably, the seal is made and laid out such that the forces due to the radial pressure, the spring, and due to friction on segments generate a minimum contact pressure between the segments and the friction part, or cancel out for a specific clearance between segments and the friction part, guaranteeing the required permeability to air. There is a clear distinction between this type of seal and seals such as those described in French patent 2 688 283 which move axially in front of an opening to produce two sealing states depending on the pressure difference and which are not segmented, and also seals described in French patent 2 613 019 in which a liquid is injected through the seal to separate the segments from the shaft. The latter seal requires a special high pressure supply circuit in order to inject the liquid and to retrieve it.